Process and apparatus for the recovery of krypton and/or xenon

ABSTRACT

Krypton and/or xenon is separated crudely from a mixture comprising oxygen and at least one rare gas selected from the group consisting of krypton and xenon in a process comprising feeding said mixture or a mixture derived therefrom to a rare gas recovery system and separating said mixture feed in said rare gas recovery system into rare gas-lean gaseous oxygen (“GOX”) and rare gas-enriched product. The process is characterized in that at least about 50 mol % of said mixture is fed to the rare gas recovery system in the gaseous phase provided that, when said mixture feed is separated by selective adsorption, the concentration of xenon in the mixture feed is no greater than 50 times the concentration of xenon in air. One advantage of a preferred embodiment of the present invention is that it can easily be retrofitted to existing pumped LOX cycle ASUs.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of air separationand has particular reference to the crude recovery of at least one raregas selected from the group consisting of krypton and xenon from anoxygen product of an air separation.

Krypton and xenon are present in air at very low concentrations,typically about 1.14 parts per million (“ppm”) and about 0.087 ppmrespectively. They are both valuable gases and, thus, there is aneconomic incentive to maximise their recovery in an air separationprocess.

In typical cryogenic air distillation processes, krypton and xenonconcentrate in the liquid oxygen (“LOX”) product taken from the bottomof the low pressure (“LP”) distillation column as they are far lessvolatile than oxygen. The smaller the LOX flow, therefore, the moreconcentrated the krypton and xenon in this product.

In cryogenic air distillation processes in which most of the oxygenproduct is removed from the LP column in the gas phase, it is possibleto make sure that very little krypton and xenon is lost in the gaseousoxygen (“GOX”) by removing the GOX several distillation stages above thebottom of the LP column. These bottom guard stages are used mainly toprevent excessive loss of krypton which is substantially more volatilethan xenon. Almost all of the krypton and xenon entering the airseparation plant can then be recovered in the LOX product, which is avery small proportion of the total oxygen flow. This LOX product canthen be processed to produce a purified rare gas product. In the eventthat it is primarily a xenon product that is required, one coulddispense with the bottom guard stages and still recover much of thekrypton and almost all of the xenon entering the plant in the LOXproduct.

If the LOX flow from the distillation process is much greater, forexample when all the oxygen is withdrawn from the distillation column asLOX, pumped to the required pressure and evaporated in the main heatexchanger, the loss of krypton and xenon is much greater, even when theLOX is taken several stages up the LP column, separately from the liquidstream in which is concentrated the rare gas components. Essentially allof the krypton and xenon entering the air separation plant flows downthe LP column to the sump of the LP column in the descending liquid, soany liquid withdrawal will remove a portion of the krypton and xenonproportional to the total liquid withdrawn as product. This willtypically lead to losses of about 30% of these valuable products.

It is desirable, therefore, to increase the recovery of krypton andxenon from an air separation plant in which at least part of the oxygenproduct is withdrawn as LOX.

When a plant withdraws the main oxygen product as a vapor from the LPcolumn, the krypton and xenon can be recovered by processing the LOXproduct as described above. However, for existing pumped LOX cycleplants, there is usually no small stream having concentrated rare gascomponents as all oxygen products are generally withdrawn from thebottom of the LP column. Therefore, as krypton and xenon are sovaluable, it is also desirable to be able to retrofit rare gas recoverysystems to existing plants.

In addition, it is desirable to provide a xenon and/or krypton recoveryplant, which can process rare gas-enriched feed streams from an externalsource.

U.S. Pat. No. 4,805,412 (Colley; published on Feb. 21, 1989) discloses aprocess and apparatus for the cryogenic distillation of air with reducedloss of krypton and xenon. Oxygen is withdrawn from the LP column of thedistillation system and is fed to a primary krypton column forextraction of its krypton and xenon content. The main feed to theprimary krypton column is a stream of LOX but a small stream of GOX isalso taken from the LP column and is fed without pressure adjustment tothe krypton column. The LP column and the primary krypton column operateat substantially the same pressure. A portion of the krypton-leanoverhead vapor is condensed and fed to the primary krypton column asdescending wash liquid.

U.S. Pat. No. 6,301,929 (Lochner; published on Oct. 16, 2001) disclosesan air separation process in which a rare gas-lean LOX stream and a raregas-enriched LOX stream are formed. The two liquid streams are pumped toa rare gas separation column operating at GOX product pressure. The raregas-lean LOX stream is passed as reflux to the top of the column and therare gas-enriched stream is passed to a lower section of the column.Rare gas-lean GOX product is withdrawn as overhead from the column and afurther rare gas-enriched bottoms liquid stream is withdrawn. Thereboiler/condenser in the sump of the column is sized to vaporize almostall the oxygen feed streams. As the oxygen feed streams are liquid, thereboiler/condenser must be large to vaporize all of the feed.

Research Disclosure No.42517 (disclosed anonymously in September 1999)discloses an air separation process in which the oxygen product isremoved from the column system as LOX. The LOX stream is pumped to theoxygen product pressure and divided into two steams. The first stream ispassed as reflux to the top of a rare gas column and the second streamis passed to a lower zone of the column. The relative proportions of thetwo streams are determined such that the column can reject methane. Raregas-lean GOX product is withdrawn as overhead from the rare gas columnand a rare gas-enriched bottoms liquid stream is withdrawn. Thereboiler/condenser in the sump of the rare gas column must be sized tovaporize almost all the oxygen feed streams. As the oxygen feed streamsare liquid, the reboiler/condenser must be large to vaporize all of thefeed.

DE-A-19855485 (Lochner; published on Jun. 10, 1999) discloses an airseparation process in which rare gas-lean LOX and rare gas-enriched LOXare formed in the LP column. The two liquid streams are pumped to a raregas column, the lean stream being passed as reflux to the top of thecolumn and the enriched stream being passed to a lower section of thecolumn. In addition, some gaseous nitrogen (“GAN”) is added to thebottom of the rare gas column to strip liquid descending the column.Rare gas-lean GOX overhead is returned to the LP column and a furtherrare gas-enriched LOX stream is withdrawn.

U.S. Pat. No. 6,378,333(Dray; published on Apr. 30, 2002) discloses anair separation process in which a first LOX stream having a xenoncomponent is passed from the LP column to the upper portion of a xenonconcentrator column as reflux. In the xenon concentrator column, the LOXfeed is separated into xenon-rich bottoms liquid and xenon-lean GOXoverhead. A second LOX stream having a xenon component is withdrawn fromLP column, pressurized and partially vaporized against a portion of feedair. Typically, the liquid fraction from this partial vaporization willbe also passed as feed to the xenon concentrator column.

U.S. Pat. No. 5,913,893 (Gary et al; published on Jun. 22, 1999)discloses a method of purification of a cryogenic fluid, especiallyliquid helium, by filtration and/or adsorption. The impurities arefiltered/adsorbed from the fluid but are not available as a valuableproduct.

U.S. Pat. No. 5,039,500 (Shino et al; published on Aug. 13, 1991)discloses the gasification of a small LOX purge stream taken from an airseparation unit (“ASU”) and passing the gaseous stream through anadsorber which selectively adsorbs xenon. Xenon is recovered during theregeneration phase of the adsorber. The xenon concentration in the purgestream is about 31 ppm, i.e. about 360 times the concentration of xenonin air.

JP-A-09002808 (Takano et al; published on Jan. 7, 1997) discloses thegasification of a small LOX purge stream taken from an ASU and passingthe gaseous stream through a first adsorber (which selectively adsorbsxenon) and then through a second adsorber (which selectively adsorbskrypton). Xenon and krypton are recovered during the regeneration phaseof the adsorbers.

It is well known in the art that krypton and xenon will concentrate inoxygen liquid because of the extremely low volatility of these gases.Thus, it is a requirement in the prior art that a LOX stream beprocessed in order to recovery krypton and xenon. Most of the prior artadditionally provides a small oxygen purge stream concentrated inkrypton and xenon so that the crude recovery system will be smaller.There is no disclosure in the aforementioned prior art of the recoveryof krypton and xenon from a warm product gaseous oxygen stream.

In prior art processes, if krypton and xenon recovery is required, it isgenerally necessary to design the LP column of an ASU so that a smallrare gas-rich LOX purge can be withdrawn. Such modifications addsignificantly to the necessary capital investment and to the height tothe LP column.

It is desirable to overcome disadvantages of (and thereby improve on)the exemplified prior art and to provide an air separation process thatis able to produce a rare gas-enriched product (for further processinginto purified krypton and/or xenon products) and a pure LOX productwithout involving the capital expense and running costs of a largereboiler/condenser or additional equipment such as an argon strippingcolumn.

BRIEF SUMMARY OF THE INVENTION

The inventors have realized that crude xenon recovery can be achieved bycontacting xenon- (and usually krypton-) containing vapor feed with areflux liquid, even if the vapor feed has a low concentration of kryptonand xenon and if the vapor feed is at a high pressure. Krypton can alsobe recovered. The recovered product may then be further processed toprovide at least one purified krypton and/or xenon product. Theexpression “low concentration” used in the context of krypton and xenonin the vapor feed is intended to mean that the krypton and xenonconcentration in the vapor feed is lower than in prior art purge streamsbut higher than in air.

According to a first aspect of the present invention, there is provideda process for the recovery of at least one rare gas selected from thegroup consisting of krypton and xenon from a mixture comprising oxygenand at least one rare gas selected from the group consisting of kryptonand xenon. The process comprises feeding said mixture or a mixturederived therefrom to a rare gas recovery system and separating saidmixture feed in said rare gas recovery system into rare gas-lean GOX andrare gas-enriched product. The process is characterised in that at leastabout 50 mol % of said mixture is fed to the rare gas-recovery system inthe gaseous phase. When the mixture feed is separated by selectiveadsorption, the concentration of xenon in the mixture feed is no greaterthan 50 times the concentration of xenon in air.

The invention involves passing feed mixture, the bulk of which is oxygenand at least about half of which is gaseous, to a crude rare gasrecovery system. The crude rare gas recovery system could be a column, acolumn system, a heat exchanger or an adsorber but, whatever the natureof the recovery system, krypton and/or xenon components are concentratedand the concentrated product (and purified oxygen product) recovered.Rather than processing a small purge stream rich in krypton and xenon,preferred embodiments of the invention are intended to process a largerGOX stream having a lower krypton and xenon concentration.

The feed is preferably at a pressure higher than that of the source fromwhich it is taken, e.g. the cryogenic air distillation column from whichit was originally withdrawn. The feed may be vaporized oxygen resultingfrom a pumped LOX cycle in an ASU, GOX from the warm-end of the mainexchanger (possibly following compression) or may be from an oxygenpipeline. LOX may also be fed to the crude recovery system to providerefrigeration and/or reflux.

One advantage of the present invention is that it can be applied toexisting plants producing oxygen. Such plants may be easily retrofittedwith a crude recovery system according to the present invention suchthat the krypton and xenon in the existing oxygen product streams may berecovered without additional processing.

Preferably, at least 90 mol % of the mixture feed is gaseous. Morepreferably, all of the mixture feed is gaseous.

The process is applicable to the production of xenon-enriched product,krypton-enriched product and xenon- and krypton-enriched product.

The process may further comprise feeding a xenon-enriched stream to therare gas-recovery system. The enriched stream may be at least partiallygaseous or may be liquid and may be taken from a cryogenic airdistillation system, which would usually be a different system to thatgenerating the mixture.

The mixture may be taken from a GOX pipeline in which case it wouldusually be under pressure and may not further require pressurization.Alternatively, the mixture may be pressurized before being fed to therare gas recovery system, for example, if it is removed from a LP columnof an ASU.

The process may further comprise separating feed air in a cryogenic airseparation unit (“ASU”) into nitrogen-rich overhead vapor and liquidoxygen (“LOX”), pressurising at least a portion of said LOX to providepressurized LOX and at least partially vaporizing at least a portion ofsaid pressurized LOX to provide said mixture feed. In such processes,all of said at least a portion of said pressurized LOX is preferablyvaporized to produce said mixture. The pressure of the mixture feed ispreferably greater than the operating pressure of the part of the ASUproducing said LOX. The LOX stream may be divided into at least twoportions, each portion being vaporized at different pressures beforebeing fed to the rare gas recovery system.

In one embodiment of the invention, the rare gas recovery system is agas-liquid contact separation system and the process comprisescontacting said mixture feed with LOX in the separation system to effectthe separation.

In one arrangement of this embodiment, the gas-liquid contact separationsystem is a gas-liquid contact column with no distillation stages andthe process comprises passing (e.g. bubbling) the mixture feed throughLOX in said column to effect the separation.

In another arrangement, the gas-liquid contact separation system is adistillation system and the process comprises feeding said mixture tothe distillation system for separation into said rare gas-lean GOX asoverhead vapor and said rare gas-enriched product and feeding LOX tosaid distillation system as reflux. Preferably, the mixture issuperheated prior to being fed to the distillation system.

Whether at least the majority of the mixture is fed to the distillationsystem in either gaseous or liquid state, the rare gas-lean overhead maybe condensed, pressurised to a high pressure (e.g. using a pump) andthen revaporised.

Where the gas-liquid contact separation system is a distillation system,the process may further comprise separating feed air in a cryogenic ASUinto nitrogen-rich overhead vapor and LOX, removing a stream of LOX fromthe ASU, pressurizing at least a portion of said LOX stream to produce astream of pressurized LOX, dividing said pressurized LOX steam into amajor portion and a minor portion, at least partially vaporising saidmajor portion to provide said mixture and feeding said minor LOX portionto the distillation system as reflux. All of said major portion ispreferably vaporized to produce said mixture. Where the distillationsystem comprises a single distillation column, the process comprisesfeeding said mixture to the column for separation into said raregas-enriched product and said rare gas-lean GOX and feeding said minorLOX portion to said column as reflux.

Where the gas-liquid contact separation system is a distillation system,the process may comprise separating feed air in a cryogenic ASU intonitrogen-rich overhead vapor and LOX, removing a stream of LOX from theASU, pressurizing at least a portion of said LOX stream to produce astream of pressurized LOX, at least partially vaporising at least aportion of said pressurized LOX steam to provide said mixture,condensing at least a portion of said rare gas-lean GOX overhead vaporby indirect heat exchange against a refrigerant to produce condensedoverhead and feeding at least a portion of the condensed overhead to thedistillation system as reflux. Preferably, all of said at least aportion of said pressurized LOX stream is vaporized to produce saidmixture. If the distillation system comprises a single distillationcolumn, then the process may comprise feeding said mixture to the columnfor separation into said rare gas-enriched product and said raregas-lean GOX and feeding at least a portion of said condensed overheadto said column as reflux.

The distillation system may comprise one or more distillation columns.Usually, the system has only one column as this reduces the initialcapital investment but systems having either two or three columns may bepreferred in certain circumstances.

In one arrangement of this embodiment, the distillation system comprisesat least a higher pressure (“HP”) distillation column and a lowerpressure (“LP”) distillation column. The HP and LP columns are thermallyintegrated via a reboiler/condenser. The process comprises feeding saidmixture to said HP column where it is separated into rare gas-depletedoverhead vapor and rare gas-enriched bottoms liquid. Rare gas-enrichedbottoms liquid is fed to said LP column after pressure adjustment forseparation into said rare gas-lean GOX and said rare gas-enrichedproduct. Rare gas-depleted overhead vapor is at least partiallycondensed by indirect heat exchange against rare gas-enriched product toproduce at least partially condensed rare gas-depleted overhead, atleast a portion of which is fed to the HP column as reflux. Liquid fromor derived from the HP column is fed to the LP column as reflux. LOX maybe fed to the HP column as reflux. LOX for reflux is typically producedby the separation of feed air in a cryogenic ASU. The HP column may bereboiled by at least partially vaporizing rare gas-enriched bottomsliquid by indirect heat exchange against a heating fluid.

In another arrangement of this embodiment, the distillation systemcomprises at least a higher pressure (“HP”) distillation column, amedium pressure (“MP”) distillation column and a lower pressure (“LP”)distillation column. The HP and MP columns are thermally integrated viaa first reboiler/condenser and the MP and LP columns are thermallyintegrated via a second reboiler/condenser. The process comprisesfeeding said mixture to said HP column where it is separated into firstrare gas-depleted overhead vapor and first rare gas-enriched bottomsliquid. First rare gas-enriched bottoms liquid is fed to said MP columnafter pressure adjustment for separation into second rare gas-depletedoverhead vapor and second rare gas-enriched bottoms liquid. First raregas-depleted overhead vapor is at least partially condensed by indirectheat exchange against second rare gas-enriched bottoms liquid to produceat least partially condensed first rare gas-depleted overhead, at leasta portion of which is fed to the HP column as reflux. Liquid from orderived from the HP column is fed to the MP column, the LP column orboth as reflux. Second rare gas-enriched bottoms liquid is fed to saidLP column for separation into said rare gas-lean GOX and said raregas-enriched product. Second rare gas-depleted overhead vapor is atleast partially condensed by indirect heat exchange against said raregas-enriched product to produce at least partially condensed second raregas-depleted overhead, at least a portion of which is fed to the MPcolumn as reflux. Liquid from or derived from the MP column is fed tothe LP column as reflux. The process may further comprise feeding LOX tothe HP column as reflux. The LOX for this reflux is preferably producedby cryogenic separation of feed air.

In a further arrangement of this embodiment, the distillation systemcomprises at least a first distillation column and a second distillationcolumn with first and second columns operating at the same pressure. Theprocess comprises feeding said mixture to said first column forseparation into rare gas-lean GOX and rare gas-enriched product, feedingsaid mixture to said second column for separation into rare gas-lean GOXand rare gas-enriched product, feeding LOX to said first column asreflux and feeding at least one liquid selected from the groupconsisting of rare gas-enriched product from the first column and LOX tosaid second column as reflux. The process may further comprise dividinga stream of said pressurized mixture into two equal portions and feedingone portion to each column.

In another embodiment, the gas-liquid contact separation system is atleast one heat exchanger. In such an embodiment, the process comprisesfeeding said mixture to the bottom of the or each heat exchanger,condensing a portion of said mixture ascending through the passages ofthe or each heat exchanger by indirect heat exchange against refrigerantto produce condensed mixture and contacting ascending mixture withdescending condensed mixture in the passages to effect the separation bydephlegmation. Preferably, the indirect heat exchange takes place in theupper portion of the or each heat exchanger. The or each heat exchangermay be reboiled by at least partially vaporizing rare gas-enrichedproduct by indirect heat exchange against a first heating fluid. Theprocess may further comprise warming the rare gas-lean GOX to ambienttemperature by indirect heat exchange within the or each heat exchangeragainst a second heating fluid, said heat exchange taking place abovethe heat exchange to produce the condensed mixture.

In a third embodiment, the rare gas recovery system is an adsorbersystem and the process comprises contacting said mixture feed with raregas selective adsorbent material in the adsorber system to effect theseparation. The process may be either a pressure swing adsorption(“PSA”) process or a temperature swing adsorption (“TSA”) process, bothof which are well-known in the art.

The concentration of xenon in the mixture feed is no more than 50 times,preferably no more than 20 times and most preferably about 5 times, theconcentration of xenon in air.

The separation is usually a crude separation and the rare gas-enrichedproduct may be further processed to produce at least one productselected from the group consisting of a purified xenon product, apurified krypton product and a purified krypton and xenon product. Suchfurther processing steps are well known in the art and includecombusting the rare gas-enriched product to remove hydrocarbon compoundsfollowed by further purification of the resultant product bydistillation.

At least one further adsorber may be used to remove hydrocarboncompounds, carbon dioxide and/or nitrous oxide from LOX or GOX feedstreams to the rare gas recovery system or from intermediate or finalLOX or GOX streams.

According to a second aspect of the present invention, there is providedapparatus for the recovery of at least one rare gas selected from thegroup consisting of krypton and xenon from a mixture comprising oxygenand at least one rare gas selected from the group consisting of kryptonand xenon according to the first aspect, said apparatus comprising acryogenic ASU for separating feed air into nitrogen-rich overhead vaporand LOX, ressurizing means for pressurizing at least a portion of saidLOX to provide pressurized LOX, vaporizing means for vaporizing at leastabout 50 mol % of said pressurized LOX to provide said mixture, and arare gas recovery system for separating said mixture into rare gas-leanGOX and rare gas-enriched product.

The rare gas recovery system may be a gas-liquid contact column with nodistillation stages. The mixture is passed (e.g. bubbled) through LOX insuch a column.

The rare gas recovery system may also be a distillation system. Suchapparatus may further comprise means for superheating said mixture priorto it being fed to the distillation system. The apparatus may furthercomprise conduit means for feeding a portion of said pressurized LOXfrom the pressurizing means (e.g. a pump) to the vaporizing means andfurther conduit means for feeding the remaining portion of saidpressurized LOX from the pump to the distillation system as reflux. Theapparatus may further comprise heat exchange means for at leastpartially condensing a portion of said rare gas-lean GOX overheadagainst a refrigerant to provide at least partially condensed raregas-lean GOX overhead and conduit means for feeding at least a portionof said at least partially condensed overhead to the distillation systemas reflux.

In a preferred embodiment where the rare gas recovery system is adistillation system, the apparatus may further comprise a firstdistillation column for separating mixture into rare gas-lean GOX andrare gas-enriched product, a second distillation column for separatingmixture into rare gas-lean GOX and rare gas-enriched product at the samepressure as said first distillation column, conduit means for feedingLOX to the first distillation column as reflux and conduit means forfeeding at least one liquid selected from the group consisting of raregas-enriched product from the first distillation column and LOX to thesecond distillation column as reflux.

The rare gas recovery system may be at least one heat exchanger forseparating the mixture by dephlegmation. The apparatus may furthercomprise first heat exchange means provided in the upper portion of theor each heat exchanger for condensing ascending mixture by indirect heatexchange against a refrigerant. The apparatus may further comprisesecond heat exchange means, e.g. provided in the lower portion of the oreach heat exchanger, for vaporizing rare gas-enriched product byindirect heat exchange against a first heating fluid. The apparatus mayfurther comprise third heat exchange means provided above the first heatexchange means in the or each heat exchanger for warming rare gas-leanGOX to ambient temperature by indirect heat exchange against a secondheating fluid.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a schematic representation of an embodiment of the presentinvention in which the rare gas recovery system is a single distillationcolumn;

FIG. 1B is a schematic representation of a different arrangement of theembodiment of the present invention depicted in FIG. 1A;

FIG. 2A is a schematic representation of another arrangement of theembodiment of the present invention depicted in FIG. 1;

FIG. 2B is a schematic representation of a different arrangement of theembodiment of the present invention depicted in FIG. 2A;

FIG. 3 is a schematic representation of the distillation column depictedin FIGS. 1A, 1B, 2A and 2B without the optional bottom section ofdistillation stages;

FIG. 4 is a schematic representation of an embodiment of the presentinvention in which the rare gas recovery system has two distillationcolumns operating at different pressures;

FIG. 5 is a schematic representation of a different arrangement of theembodiment of the present invention depicted in FIG. 4;

FIG. 6 is a schematic representation of an embodiment of the presentinvention in which the rare gas recovery system has three distillationcolumns, each column operating at a different pressure;

FIG. 7 is a schematic representation of an embodiment of the presentinvention in which the rare gas recovery system has two distillationcolumns operating at the same pressure;

FIG. 8 is a schematic representation of an embodiment of the presentinvention in which the rare gas recovery system is a heat exchanger;

FIG. 9 is a schematic representation of a different arrangement of theembodiment of the present invention depicted in FIG. 8; and

FIG. 10 is a schematic representation of an embodiment of the presentinvention in which the rare gas recovery system comprises an absorber.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1A, LOX is withdrawn as stream 100 from the LP column10 of a double column ASU. The LOX stream 100 is pumped by LOX pump 12to provide a pumped LOX stream 102 which contains essentially all thekrypton and xenon which has entered the double column ASU. The kryptonand xenon concentration in the LOX is about 5 times greater than that inatmospheric air as the flow of stream 100 is typically 20% of the ASUfeed air flow. The pumped LOX stream 102 is divided into two portions.The major portion 106 is vaporized to at least a quality of 0.9 (i.e.90% of the stream is gaseous) in the heat exchanger 14 against acondensing compressed air stream 130. Ideally, the major portion 106 iscompletely vaporized and slightly superheated. The resultant vaporizedstream 108 is passed to a lower zone of a single crude rare gas recoverycolumn 16. The minor portion 104 of the pumped LOX is passed as refluxto the top of the crude column 16. The flow of reflux stream 104 isdetermined by the desired recovery of krypton and xenon.

In the crude distillation column 16, separation is effected intooverhead rare gas-lean GOX, which is withdrawn as stream 110 and warmedin the heat exchanger 14 to provide GOX stream 112, and raregas-enriched bottoms liquid, which is withdrawn as product stream 120and which can be further purified by known methods.

In heat exchanger 14, the LOX stream 106 is warmed and vaporized againsta compressed feed air stream 130 (although other pressurized streamssuch as high pressure nitrogen could be used instead). Most of thecompressed feed air stream 130 is withdrawn from the heat exchanger 14as a stream 136 of liquefied air but a portion 132 is withdrawn from theheat exchanger 14 and liquefied in crude column reboiler 18 to provide afurther stream 134 of liquified air. The two liquefied air streams arecombined and the combined stream 138 is fed to the ASU double columnsystem.

It should be noted that it is not necessary to the invention that theheating fluid for reboiler 18 should have the same composition as thefluid condensed by the boiling LOX in heat exchanger 14. In addition,the reboiler 18 may be provided outside the column 16 rather than in thesump of the column as shown. Further, if reboiler 18 is provided outsidecolumn 16, then it may be fed directly with liquid from column 16 or itmay be located inside a pot fed by liquid from column 16. It should alsobe noted that, provided oxygen feed stream 108 is sufficientlysuperheated, then column 16 may not require a reboiler. The descendingreflux liquid would then be largely vaporized by desuperheating of thefeed stream 108.

Higher reflux flows in column 16 more easily wash the krypton and xenoncomponents from the rising oxygen vapor. As stated, it is preferred thatstream 108 should be completely gaseous. One reason for this is that anyliquid in stream 108 will have to be vaporized in the reboiler 18 whichnecessitates increasing the size of the reboiler.

Crude column 16 is shown having two distillation sections. In the uppersection, krypton and xenon are washed from the rising gas by the refluxliquid. The lower section is optional and serves to increase theconcentration of krypton and xenon in the bottoms liquid. Althoughreflux for the crude column is shown as provided by LOX feed stream 104,this source of reflux could be supplemented by or replaced by use of acolumn condenser, driven by an appropriate refrigerant.

In principle, column 16 does not need any distillation stages at all.Oxygen feed stream 108 may be bubbled through liquid in the sump ofcolumn 16. Much of the xenon in the oxygen feed would transfer to thesump liquid. If the oxygen feed 108 is two-phase, then most of the xenonin the feed would be in the liquid phase. However, having at least onedistillation stage, in addition to the reboiler, is highly preferred.

An optional rare gas-enriched stream 140, which may originate from adistillation column different from that from which the oxygen feedstream 108 originates, may be fed to the column 16. Typically, ifpresent, stream 140 may be a rare gas-rich purge stream from a differentair separation plant and it may be liquid or at least partially gaseous.This optional rare gas-enriched feed stream is applicable to anyembodiment of the invention.

The process depicted in FIG. 1B is similar to that depicted in FIG. 1Aand the corresponding features have been given the same numericallegends.

In the crude distillation column 16, separation is effected intooverhead rare gas-lean GOX, and rare gas-enriched bottoms liquid, whichis withdrawn as product stream 120 and which can be further purified byknown methods. The overhead rare gas-lean GOX is condensed in overheadcondenser 20. A portion 109 of the resulting liquid is returned to thedistillation column 16 as reflux and the remaining portion 111 is pumpedto a higher pressure (preferably supercritical) in pump 13, vaporised(if sub-critical) and warmed in the heat exchanger 14 to provide GOXstream 112. The warming (and evaporation if present) of this GOX streamis primarily against compressed air stream 160, which is cooled (and, ifsub-critical condensed) to form stream 162 which is fed to the ASUdouble column system.

In heat exchanger 14, the LOX stream 106 is warmed and vaporized againsta compressed feed or recycle air stream 130 (although other pressurizedstreams such as high pressure nitrogen could be used instead). Most ofthe compressed air stream 130 is withdrawn from the heat exchanger 14 asa stream 136 of liquefied air but a portion 132 is withdrawn from theheat exchanger 14 and liquefied in crude column reboiler 18 to provide afurther stream 134 of liquified air. The two liquefied air streams arecombined and the combined stream 137 is reduced in pressure and fed tothe overhead condenser 20 of the distillation column 16. Here it ispartially evaporated to produce a vapour stream 150 which is warmed inheat exchanger 14 to form stream 152. The remaining liquid 138 is fed tothe ASU double column system. The vapour stream 152 may be recompressedto form all or part of the compressed air feed stream 130

It should be noted that it is not necessary to the invention that theheating fluid for reboiler 18 should have the same composition as thefluid condensed by the boiling LOX in heat exchanger 14 or the coolingfluid for condenser 20. It should also be noted that condenser 20 neednot be located on the column, and its duty could be performed in themain heat exchanger 14.

Referring now to FIG. 2A, a stream 200 of LOX is withdrawn from a pumpedLOX ASU coldbox 20 and pumped by LOX pump 22 to provide a pumped LOXstream which is divided into two portions. The major portion isvaporized within the coldbox 20 to provide a stream 204 of warmpressurized GOX. This stream contains most of the krypton and xenonwhich entered the pumped LOX ASU. The krypton and xenon concentration inthe GOX is about 5 times higher than in atmospheric air as the flow ofstream 200 is typically 20% of the ASU feed air flow. GOX stream 204 iscooled in heat exchanger 24 to provide stream 208 having a quality of atleast 0.9. Ideally, stream 208 should be superheated. Stream 208 is fedto a lower section of a single crude rare gas recovery column 26. Theremaining portion 202 of the pumped LOX stream from pump 22 isoptionally warmed in heat exchanger 29 and fed as stream 203 to the topof column 26 to provide reflux and refrigeration.

In the crude distillation column 26, separation is effected intooverhead rare gas-lean GOX, which is removed as stream 210 and warmed inheat exchanger 24 to provide GOX product stream 212, and raregas-enriched bottoms liquid, which is withdrawn as a product stream 220and which can be further purified by known methods.

A minor portion 230 of the pumped LOX ASU compressed feed air is cooledin heat exchanger 24 and the cooled stream 232 is fed to reboiler 28where it is condensed. A stream 234 of condensed air is returned to thepumped LOX ASU, optionally after exchanging heat with LOX stream 202 insubcooler 29. Reboiler 28 may be located outside column 26, possiblyinside its own pot if desired. Stream 230 does not have to be a boostedair stream. Any suitable pressurised stream (e.g. boosted nitrogen)could be used to provide the heating duty for reboiler 28. Further, theboosted stream providing the reboiler heating duty could be supplieddirectly to the reboiler as a cold stream from the main ASU 30, ratherthan a warm stream which is cooled in exchanger 24. Exchanger 24 mighthave other streams associated with it, e.g. stream 220 might bevaporised and warmed in said exchanger prior to its furtherpurification.

The flow of reflux stream 203 is determined by the desired recovery ofkrypton and xenon. As in FIG. 1, it is preferred that stream 208 becompletely gaseous as any liquid in stream 208 would have to bevaporized by the reboiler 28 which would necessitate increasing the sizeof the reboiler.

Column 26 is depicted as having two distillation sections. In the uppersection, krypton and xenon are washed from the rising gas by the refluxliquid. The lower section is optional and serves to increase theconcentration of krypton and xenon in the bottoms liquid. Althoughreflux for the crude column is shown as LOX feed stream 203, this sourceof reflux could be supplemented by or replaced by the use of a columncondenser, driven by an appropriate refrigerant.

It can be readily appreciated from this figure that the invention issuited to the simple retrofit of a krypton and xenon recovery system toan existing pumped LOX ASU. GOX stream 204 and the compressed feed airstream 230 are both warm streams and, thus, are readily accessible forsuch a retrofit. The LOX stream 202 for reflux and refrigeration isreadily supplied by connection outside the coldbox to the discharge lineof the LOX pump 22. The compressed air, liquefied in the crude recoveryequipment, is sent as line 234 to the ASU coldbox. It is simple todesign the coldbox to easily route this stream to join the main ASUliquefied compressed feed air stream. Thus, retrofit of a rare gasrecovery system is readily available for minimal initial capitalinvestment.

The process depicted in FIG. 2B is similar to that depicted in FIG. 2Aand corresponding features have been given the same numerical legends. Astream 200 of LOX is withdrawn from a pumped LOX ASU coldbox 20 andpumped by LOX pump 22 to provide a pumped LOX stream which is dividedinto two portions. The major portion is vaporized within the coldbox 20to provide a stream 204 of warm pressurized GOX, which may be at asupercritical pressure. This stream contains most of the krypton andxenon which entered the pumped LOX ASU. The krypton and xenonconcentration in the GOX is about 5 times higher than in atmospheric airas the flow of stream 200 is typically 20% of the ASU feed air flow. GOXstream 204 is cooled (and condensed if sub-critical) in heat exchanger24 to provide stream 205 which is a liquid or supercritical dense-phasestream. A portion 250 of stream 205 is mixed with LOX stream 202 and theremaining portion 207 is reduced in pressure and evaporated in the heatexchanger 24 primarily by condensing compressed air stream 230 to formstream 208 having a quality of at least 0.9. Ideally, stream 208 shouldbe superheated. Stream 208 is fed to a lower section of a single cruderare gas recovery column 26. The remaining portion 202 of the pumped LOXstream from pump 22 is mixed with LOX stream 250, optionally heated inheat exchanger 24, reduced in pressure and fed as stream 203 to the topof column 26 to provide reflux and refrigeration.

In the crude distillation column 26, separation is effected intooverhead rare gas-lean GOX, which is removed as stream 210 and raregas-enriched bottoms liquid, which is withdrawn as a product stream 220and which can be further purified by known methods. GOX stream 210 iscondensed in heat exchanger 24, pumped to pressure in pump 23 and thepumped stream 211 re-warmed (and evaporated if sub-critical) in heatexchanger 24 primarily against the incoming GOX stream 204 to provideGOX product stream 212. The pressure of stream 212 is similar to stream204, and may, if the streams are supercritical, be higher.

Stream 230 of the pumped LOX ASU compressed feed air is split into twoportions, a minor portion 231 and a major portion 260. Stream 231 iscooled in heat exchanger 24 and the cooled stream 232 is fed to reboiler28 where it is condensed to form stream 234, Stream 260 is cooled andcondensed in heat exchanger 24, primarily by evaporating oxygen stream207, to form stream 262, which is mixed with stream 234 and reduced inpressure to form stream 264. This stream is evaporated and warmed inheat exchanger 24 to form stream 266, primarily by condensing GOX stream210 and cooling air streams 260 and 261. Reboiler 28 may be locatedoutside column 26, possibly inside its own pot if desired. The returnair stream 266 is at a lower pressure than feed stream 230 and may berecompressed in a separate compressor (not shown) to form all or part ofstream 230, or, as shown, may be returned to the suction of thecompression stage 270 following the stage 268 from which stream 230 iswithdrawn. Stream 230 will be a major portion of the flow through thecompressor 268, so that little inefficiency is introduced by reducingthe pressure of the remaining portion which goes directly to thefollowing compression stage 270.

It can also be readily appreciated from this figure that the inventionis suited to the simple retrofit of a krypton and xenon recovery systemto an existing pumped LOX ASU. GOX stream 204 and the compressed feedair stream 230 and return air stream 266 are all warm streams and, thus,are readily accessible for such a retrofit. The LOX stream 202 forreflux and refrigeration is readily supplied by connection outside thecoldbox to the discharge line of the LOX pump 22. Thus, retrofit of arare gas recovery system is readily available for minimal initialcapital investment.

FIG. 3 depicts a crude krypton and xenon recovery column 36 that issimilar to that used in the processes depicted in FIGS. 1 and 2. Column36 differs from the columns of FIGS. 1 and 2 in that the optional bottomsection of stages are omitted.

The main rare gas-containing oxygen feed to column 36 is stream 308which is at least 90% gaseous. The oxygen feed is separated in column 36into rare gas-lean GOX overhead, which is removed as stream 310, andrare gas-enriched LOX bottoms, which is removed as stream 320. Column 36is refluxed by a stream 304 of LOX fed to the top of the column. Theflow of LOX reflux stream 304 determines the losses of xenon andparticularly of the more volatile krypton in the overhead vapor.Selection of the LOX reflux/feed GOX flow determines the liquid to vapor(“L/V”) flow ratio.

It should be noted that, in FIG. 3, stream 304 is typically a fractionof the pumped LOX and thereby contains the same krypton and xenonconcentrations as the feed GOX stream 308. Thus, the overhead GOX stream310 will be in equilibrium with LOX stream 304 and so will contain somekrypton and xenon. It is within the scope of the invention to replace orsupplement stream 304 by use of a column condenser in which part of theoverhead gas is condensed against a suitable refrigerant and returned tothe column as reflux, optionally with a small fraction withdrawn as raregas-lean LOX product.

FIG. 3 shows only a single column section but, optionally, there may bea bottom section located between feed 308 and reboiler 38. Such anadditional column section would help to concentrate the krypton andxenon in the bottom product.

A stream 332 of heating fluid, e.g. compressed feed air, is condensed inreboiler 38 against boiling rare gas-enriched liquid in the sump of thecolumn 36 providing a condensed heating fluid stream 334. It should benoted that the reboiler 38 could be deleted provided that the feedstream 308 is sufficiently superheated. The refrigeration necessary todesuperheat the feed stream would then be provided by vaporizingdownflowing LOX in the column such that only stream 320 remained.

FIG. 4 depicts a crude rare gas recovery system comprising a HP column40 thermally integrated with a LP pressure column 42 via a reboiler 44.Oxygen feed stream 400, having a vapor quality of at least 0.9 andcontaining more krypton and xenon than atmospheric air, is fed to alower portion of HP column 40. Ideally, feed stream 400 is in thegaseous state.

In HP column 40, separation is effected into overhead rare gas-depletedvapor and rare gas-enriched bottoms liquid. The enriched bottoms liquidis withdrawn as stream 404 and is fed to a lower zone of LP column 42where it is separated into rare gas-lean product, which is removed asstream 408 and further processed if required to provide purified kryptonand/or xenon products, and rare gas-enriched overhead vapor, which isremoved as stream 406. The overhead vapor from HP column 40 is at leastpartially condensed in reboiler 44 against boiling rare gas-lean productto provide at least partially condensed overhead. A portion of thecondensed overhead is used as reflux for HP column 40 and the remainderis fed as stream 402 to LP column 42 as reflux. A portion of stream 402can optionally be withdrawn as rare gas-depleted LOX product in stream412.

Typically, the flowrate of stream 402 will be approximately half of theflow leaving reboiler 44, so that the L/V ratio in both columns issimilar. The L/V ratio will be about 0.5 if oxygen feed stream 400 isall vapor as preferred. The higher the L/V ratio, the more difficult itis for xenon and particularly for krypton to escape from the columnsystem.

The pressure difference between the two columns will tend to be smallbecause the composition of fluids on both side of reboiler 44 will besimilar. Thus, there will be issues to consider in liquid transfer fromHP column 40 to LP column 42. One or more pumps might be necessary toaccomplish this transfer. Alternatively, GOX vapor could be injectedinto the transfer lines thereby using “vapor lift” to accomplish theliquid transfer.

The columns are depicted as stacked with LP column 42 located above HPcolumn 40. However, the invention also applies to arrangements where thetwo columns are side by side or even if HP column 40 were stacked aboveLP column 42.

The process represented in FIG. 4 uses only a single column section incolumns 40 and 42 but, optionally, there could be more than one section.For example, in LP column 42 there could be a bottom section betweenfeed 404 and the reboiler 44, which would help to concentrate thekrypton and xenon in the bottom product.

The approximate column L/V ratio of 0.5 available for the above schememay be higher than necessary to achieve the desired recovery of kryptonand xenon. A lower L/V ratio would result if part of the oxygen vaporfeed 400 to the crude recovery system is routed directly to a lower zoneof the LP column 42 as optional stream 401. The higher the flow of LPcolumn GOX feed stream 401, the lower the L/V ratio in the columns.

For a pumped LOX ASU, such as that shown in FIG. 1, the pumped oxygen isnormally vaporized at a single pressure in the main exchanger. It would,however, be possible to divide the pumped LOX into two streams andvaporize the two streams at slightly different pressures for minimalpower penalty. Such an arrangement would taking advantage of the factthat compressed air condenses over a temperature range (as it is amixture of gases). Usually, there would be no incentive to takeadvantage of this capability, as customers do not want oxygen product attwo slightly different pressures. However, such a capability wouldideally match the process of FIG. 4 in which part of the GOX feed 401can be supplied at a slightly lower pressure. Thus, the process depictedin FIG. 4 can deliver any column L/V ratio up to about 0.5 depending ofthe flow of optional stream 401 and on the quality and/or superheat offeed stream 400.

Refrigeration for the column recovery system is most easily supplied byuse of a LOX feed stream although an external refrigerant could be usedto exchange heat with a suitable stream from the crude recovery system.The LOX refrigerant stream would typically be part of the pumped LOXstream from the main ASU and can be routed to any location of the cruderecovery column system. However, the most valuable arrangement would bewhere the LOX refrigerant is fed as stream 410 the top of HP column 40as reflux for that column, thereby allowing an increase in the flow ofreflux stream 402 to LP column 42.

A significant advantage of the arrangement depicted in FIG. 4 is that itonly requires connections for LOX and GOX and is, thus, highly suitedfor retrofit to an existing pumped LOX ASU due to the fewinterconnections required.

FIG. 5 depicts a similar arrangement to that depicted in FIG. 4 and,accordingly, the same numerical legends have been used in FIG. 5 todenote the features that correspond with those of FIG. 4. Thearrangement in FIG. 5 has a reboiler 46 for the HP column 40. A stream420 of heating fluid is used to boil the rare gas-depleted liquidbottoms in the HP column 40, thereby generating a condensed heatingfluid stream 422. For example, the heating fluid in stream 420 may be aportion of the compressed air used to vaporize the pumped LOX in themain ASU. As this external heat is added to the crude recovery columnsystem, extra refrigeration will be required as compared with theprocess of FIG. 4. The simplest way of providing this extrarefrigeration is by increasing the flow of LOX refrigerant stream 410.Stream 410 is shown as being used as reflux for HP column 40 but itcould instead be used as reflux for LP column 42. The additionalrefrigeration could also be provided by use of external refrigerant in acondenser for the LP column 42.

The column L/V ratio in FIG. 4 is limited to approximately 0.5. However,the use of the reboiler 46 in the process of FIG. 5 can deliver higherL/V ratios depending primarily on the duty of reboiler 46. Such high L/Vratios could be employed to improve krypton recovery for cases havingvery high column operating pressures.

FIG. 6 depicts an arrangement of a crude recovery system comprising a HPcolumn 60, a MP column 62 and a LP column 66. A reboiler 64 thermallyintegrates columns 60 and 62 and a reboiler 68 thermally integratescolumns 62 and 66.

Oxygen feed stream 600, containing a greater concentration of kryptonand xenon than in atmospheric air, is fed to a lower portion of HPcolumn 60. Stream 600 has a vapor quality of at least 0.9 and is,ideally, in the gaseous state. In HP column 60, separation is effectedinto a first rare gas-depleted vapor overhead and a first raregas-enriched bottoms liquid which is passed as feed stream 604 to alower zone of MP column 62. The overhead vapor from HP column 60 iscondensed in reboiler 64. A portion of the resultant condensed stream isused as reflux for HP column 60 and the remaining portion is fed asstream 602 to LP column 66 as reflux (although optionally all or part ofthis stream could be passed as reflux to MP column 62). A portion ofstream 602 can optionally be withdrawn as rare gas-depleted LOX product.

In MP column 62, separation is effected into a second rare gas-depletedoverhead vapor and a second rare gas-enriched bottoms liquid which isfed as a feed stream 606 to LP column 66. The second rare gas-depletedoverhead vapor from MP column 62 is condensed in reboiler 68. A portionof the resultant condensed stream is used as reflux for MP column 62 andthe remaining portion is fed as stream 608 to LP column 66 as reflux. Aportion of stream 608 can optionally be withdrawn as rare gas-depletedLOX product stream 612.

In LP column 66, separation is effected into rare gas-lean overheadvapor, which is removed as product stream 610, and rare gas-enrichedliquid product which is withdrawn as stream 620 and can be furtherpurified by known methods. Refrigeration for the column system can beprovided by injecting a LOX stream in the column system, for example asstream 630 into the top of HP column 60.

Typically, one third of the vapor condensed in each reboiler is passedas reflux to LP column 66 in streams 602 and 608 thereby allowing eachof the three columns to operate with an L/V ratio of approximately 0.67.In order to require such a high L/V ratio to prevent loss of krypton,the operating pressure of the columns would have to be high, for examplegreater than about 30 bar. The pressure difference between the threecolumns will tend to be small because, for each reboiler, the fluidcompositions on each side of the reboiler are similar. Thus, there maybe issues in liquid transfer between the columns to be considered. Itmay prove necessary to use pumps or to inject GOX vapor into the liquidtransfer lines (to achieve vapor lift) to accomplish the liquidtransfer.

The columns are shown are shown stacked with LP column 66 above MPcolumn 62 which is above HP column 60. However, the invention alsoapplies to arrangements where the columns are side by side or stackeddifferently, even if HP column 60 is stacked above LP column 66.

The arrangement depicted in FIG. 7 is a crude recovery system comprisingtwo columns 70 and 72 operating at the same pressure. Reboiler 74 islocated in a lower zone of column 72. This arrangement demonstrates howthe column in FIG. 3 can be reduced in diameter by processing only part(typically half) of the main rare gas-containing oxygen feed in eachcolumn. The at least mainly gaseous oxygen feed 700 is divided betweenthe two columns. GOX stream 702 is fed to a lower zone of column 70where it is contacted by reflux LOX stream 720 to form rare gas-leanGOX, which is removed as stream 704, and rare gas-enriched bottomsliquid, which is removed as stream 722. The remaining fraction of oxygenfeed stream 700 is fed as stream 706 to a lower zone of column 72.Reflux for column 72 can be provided by rare gas-enriched liquid asstream 722 and/or by LOX stream 726 (which could be a portion of oxygenfeed stream 720. Rather than being used as top reflux for column 72,stream 722 could instead be passed to a lower zone of column 72 asindicated by stream 724.

Column 72 has a reboiler 74 driven by a heating fluid 730, 732 whichcould be a portion of the compressed air used to vaporize pumped LOX inthe main ASU. In column 72, separation is effected into rare gas-leanGOX which is removed as stream 708 and combines with rare gas-leanstream 704 to provide purified oxygen stream 710, and a raregas-enriched product which is removed as stream 728 and can be furtherpurified by known methods.

Columns 70 and 72 could be separate columns rather than stacked columnsas shown. The reboiler 74 could be located outside to the column system,perhaps in its own container. Column 70 could also have its own reboilerand then stream 722 of rare gas-enriched product from column 70 could becombined with stream 728 of rare gas-enriched product from column 72.Either or both of columns 70 and 72 could have more than one contactingsection e.g. there could be a bottom section of stages in column 72below feed stream 706.

By processing half of the vapor feed in each column, the cross-sectionof the columns would be halved compared to that in FIG. 3.

The embodiment depicted in FIG. 8 is crude recovery system comprising aheat exchanger 80 and a reboiler 84. The exchanger 80 will functionlargely as a dephlegmator to contact rising GOX vapor with falling LOXto generate a rare gas-enriched bottoms liquid. The primary oxygen feedstream 800, which is at least 90% vapor, is fed to the standpipe section82 of exchanger 80 and passes up through the passages of the exchanger.It is partially condensed in the exchanger by heat transfer withrefrigerant streams 830, 832 and the condensate travels downwards,countercurrent to the rising GOX in the same exchanger passages. Adephlegmating action therefore takes place in the oxygen passages. TheLOX contacting the rising GOX effects separation into rare gas-lean GOXoverhead removed as stream 802 and rare gas-enriched LOX bottoms liquidremoved as stream 804.

A portion 808 of the bottoms liquid can be reboiled as shown in reboiler84 against a stream 820 of heating fluid (which could be part of theboosted air stream used in the main ASU to boil the pumped LOX) therebyproducing condensed stream 822. The reboiled stream 810 (which could betwo-phase or all vapor) is returned to the standpipe 82. The remainingbottoms liquid 806 can be withdrawn as product and can be furtherpurified by known methods. If the oxygen feed stream 800 weresuperheated, then the reboiler 84 may be omitted as the desuperheatingof the feed would provide the heat to vaporize some of the fallingliquid.

Refrigerant stream 830, 832 does not need to transfer heat with therising GOX over the full length of the exchanger 80. In fact, it ispreferable that this heat exchange take place only in the upper zone ofthe heat exchanger 80 as shown thereby resulting in a higher L/V flowratio for upper zone of the exchanger compared with a case when the heattransfer takes place over a longer zone. The higher L/V ratio reducesthe loss of krypton and xenon in the stream 802 of overhead gas. If theexchange of heat with the refrigerant stream 830 occurs only in theupper zone of the exchanger, then the lower zones of the exchanger 80could form an adiabatic section of the exchanger 80. Although there isno heat transfer in this section, the separation due to the contact ofLOX and GOX would still occur.

It is possible to replace or supplement the refrigerant duty byinjecting LOX into the GOX passages at upper zone of the exchanger 80.There are several known methods of accomplishing this replacement orsupplement.

In FIG. 8, the stream 802 of gaseous overhead leaves the exchanger 80 assaturated vapor, as it has just been partially condensed by refrigerantstream 830. Stream 802 will typically be warmed to ambient temperatureagainst a cooling stream in another exchanger. The additional exchangercould be incorporated as the upper zone of exchanger 80.

The arrangement depicted in FIG. 9 is similar to the arrangement of FIG.8 and, accordingly, the same numerical legends have been used in FIG. 9to depict the features that correspond with those of FIG. 8. In FIG. 9,the reboiler 84 of FIG. 8 has been incorporated into the lower zone ofexchanger 80 and a upper section has been added to exchanger 80 in whichthe purified GOX product is warmed against stream 840, 842 of heatingfluid.

The embodiment depicted in FIG. 10 is a crude recovery system comprisingan adsorber vessel 90. GOX feed stream 900 is passed through theadsorber and purified oxygen product exits as stream 902. Thetemperature of stream 900 may be cryogenic or ambient. Within adsorber90, xenon and/or krypton is adsorbed on one or more adsorber beds 92.Periodically, the adsorber must be regenerated and the adsorbed rare gascomponents recovered.

The regeneration process may include depressurization of adsorber vessel90 and passage of a small quantity of regeneration gas 910 (typicallynitrogen) through the adsorber. Regeneration outlet stream 920 isenriched in the rare gas components which have desorbed from theadsorbent and which is withdrawn as product and can be further purifiedby known methods. This method of regeneration is well-known as pressureswing adsorption (“PSA”).

The adsorber 90 can also be regenerated by the well-known method oftemperature swing adsorption (“TSA”). The regeneration stream 910 mayhave been heated in an upstream heater to provide the desorption heatfor this well-known method of regeneration. Alternatively (or inaddition), heat may be provided by heating elements located at somepoint within vessel 90, or adsorbent 92.

FIG. 10 depicts a single adsorber vessel 90. Two (or more) vessels canbe used, e.g. in parallel, to allow continuous adsorption of rare gascomponents from feed stream 910.

Optionally, in any of the above figures, one or more adsorbers may beincluded to at least partially remove hydrocarbons, carbon dioxide ornitrous oxide. Such adsorber can operate to process LOX or GOX feeds tothe recovery system, or on intermediate or final GOX or LOX streams ofthe system.

EXAMPLE

By way of an example of the invention, a simulation of the processdepicted in FIG. 1 has been carried out. In the simulation, axenon-enriched stream is recovered with only secondary importance beinggiven to krypton recovery. The results of the simulation are shown inTable 1.

TABLE 1 Stream 100 102 104 108 110 120 140 Phase liq liq liq vap vap liqFlow (kgmol/h) 10000 10000 1500 8500 9988 11.8 0.0 Pressure (bar) 1.738.6 38.6 38.6 38.6 38.6 Temp (° C.) −177.8 −173.4 −173.4 −123.3 −125.5−125.4 % O₂ 99.50 99.50 99.50 99.50 99.50 99.46 % Ar 0.50 0.50 0.50 0.500.50 0.24 ppm Xe 0.43 0.43 0.43 0.43 0.05 319 ppm Kr 5.60 5.60 5.60 5.604.62 84 ppm CH₄ 14.8 14.8 14.8 14.8 14.4 392

The results indicate that high xenon recovery is achieved despite thevery high operating pressure and the low concentration of xenon in thecrude recovery column feed. 15% of the total oxygen feed is used asreflux. The remaining portion of the total oxygen feed is the primarycolumn feed in the form of slightly superheated vapor. The volatility ofkrypton and xenon will be lower at lower pressures, which would tend toreduce the reflux LOX flow requirement.

Throughout the specification, the term “means” in the context of meansfor carrying out a function is intended to refer to at least one deviceadapted and/or constructed to carry out that function.

It will be appreciated that the invention is not restricted to thedetails described above with reference to the preferred embodiments butthat numerous modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the followingclaims.

What is claimed is:
 1. A process for the recovery of at least one raregas selected from the group consisting of krypton and xenon from amixture comprising oxygen and at least one rare gas selected from thegroup consisting of krypton and xenon, said process comprising:separating feed air in a cryogenic air separation unit (“ASU”) intonitrogen-rich overhead vapor and liquid oxygen (“LOX”); pressurising atleast a portion of said LOX to provide pressurized LOX; at leastpartially vaporizing at least a portion of said pressurized LOX toprovide said mixture such that at least about 50 mol % of said mixtureis in the gaseous phase; feeding said mixture or a mixture derivedtherefrom at a pressure greater than the pressure of the part of the ASUproducing said LOX to a rare gas recovery system; and separating saidmixture feed in said rare gas recovery system into rare gas-lean gaseousoxygen (“GOX”) and rare gas-enriched product, provided that, when saidmixture feed is separated by selective adsorption, the concentration ofxenon in the mixture feed is no greater than 50 times the concentrationof xenon in air.
 2. The process according to claim 1 wherein at least 90mol % of said mixture feed is gaseous.
 3. The process according to claim1 wherein all of said mixture feed is gaseous.
 4. The process accordingto claim 1 wherein the rare gas-enriched product is enriched with xenon.5. The process according to claim 1 wherein the rare gas-enrichedproduct is enriched with krypton.
 6. The process according to claim 1wherein the rare gas-enriched product is enriched with both xenon andkrypton.
 7. The process according to claim 1 further comprising feedinga xenon-enriched stream to the rare gas-recovery system.
 8. The processaccording to claim 7 wherein the xenon-enriched stream is at leastpartially gaseous.
 9. The process according to claim 7 wherein thexenon-enriched stream is liquid.
 10. The process according to claim 1wherein the pressure of the mixture is increased before being fed to therare gas recovery system.
 11. The process according to claim 1 whereinall of said at least a portion of said pressurized LOX is vaporized toproduce said mixture.
 12. The process according to claim 1 wherein astream of said LOX is divided into at least two portions, each portionbeing vaporized at different pressures before being fed to the rare gasrecovery system.
 13. The process according to claim 1 wherein the raregas recovery system is a gas-liquid contact separation system, saidprocess further comprising contacting said mixture feed with LOX in theseparation system to effect the separation.
 14. The process according toclaim 13 wherein the gas-liquid contact separation system is agas-liquid contact column with no distillation stages, said processcomprising passing the mixture feed through LOX in said column to effectthe separation.
 15. The process according to claim 13 wherein thegas-liquid contact separation system is a distillation system, theprocess comprising feeding said mixture to the distillation system forseparation into said rare gas-lean GOX as overhead vapor and said raregas-enriched product and feeding LOX to said distillation system asreflux.
 16. The process according to claim 15 wherein the mixture issuperheated prior to being fed to the distillation system.
 17. Theprocess according to claim 15 further comprising: separating feed air ina cryogenic ASU into nitrogen-rich overhead vapor and LOX; removing astream of LOX from the ASU; pressurizing at least a portion of said LOXstream to produce a stream of pressurized LOX; dividing said pressurizedLOX stream into a major portion and a minor portion; at least partiallyvaporizing said major portion to provide said mixture; and feeding saidminor LOX portion to the distillation system as reflux.
 18. The processof claim 17 wherein all of said major portion is vaporized to producesaid mixture.
 19. The process according to claim 17 wherein thedistillation system comprises a single distillation column, said processcomprising feeding said mixture to the column for separation into saidrare gas-enriched product and said rare gas-lean GOX and feeding saidminor LOX portion to said column as reflux.
 20. The process according toclaim 15 further comprising: separating feed air in a cryogenic ASU intonitrogen-rich overhead vapor and LOX; removing a stream of LOX from theASU; pressurizing at least a portion of said LOX stream to produce astream of pressurized LOX; at least partially vaporising at least aportion of said pressurized LOX steam to provide said mixture;condensing at least a portion of said rare gas-lean GOX overhead vaporby indirect heat exchange against a refrigerant to produce condensedoverhead; and feeding at least a portion of the condensed overhead tothe distillation system as reflux.
 21. The process according to claim 20wherein all of said at least a portion of said pressurized LOX stream isvaporized to produce said mixture.
 22. The process according to claim 20wherein the distillation system comprises a single distillation column,said process comprising feeding said mixture to the column forseparation into said rare gas-enriched product and said rare gas-leanGOX and feeding at least a portion of said condensed overhead to saidcolumn as reflux.
 23. The process according to claim 15 wherein thedistillation system comprises at least a higher pressure (“HP”)distillation column and a lower pressure (“LP”) distillation column,said HP and LP columns being thermally integrated via areboiler/condenser, said process comprising: feeding said mixture tosaid HP column where it is separated into rare gas-depleted overheadvapor and rare gas-enriched bottoms liquid; feeding rare gas-enrichedbottoms liquid to said LP column after pressure adjustmnent forseparation into said rare gas-lean GOX and said rare gas-enrichedproduct; at least partially condensing rare gas-depleted overhead vaporby indirect heat exchange against rare gas-enriched product to produceat least partially condensed rare gas-depleted overhead; feeding atleast a portion of the at least partially condensed rare gas-depletedoverhead to the HP column as reflux; and feeding liquid from or derivedfrom the HP column to the LP column as reflux.
 24. The process accordingto claim 23 further comprising feeding LOX to the HP column or the LPcolumn as reflux.
 25. The process according to claim 24 furthercomprising separating feed air in a cryogenic ASU into nitrogen-richoverhead vapor and LOX and using a portion of said LOX to provide saidLOX reflux.
 26. The process according to claim 23 wherein the HP columnis reboiled by at least partially vaporizing rare gas-enriched bottomsliquid by indirect heat exchange against a heating fluid.
 27. Theprocess according to claim 15 wherein the distillation system comprisesat least a higher pressure (“HP”) distillation column, a medium pressure(“MP”) distillation column and a lower pressure (“LP”) distillationcolumn, said HP and MP columns being thermally integrated via a firstreboiler/condenser and said MP and LP columns being thermally integratedvia a second reboiler/condenser, said process comprising: feeding saidmixture to said HP column where it is separated into first raregas-depleted overhead vapor and first rare gas-enriched bottoms liquid;feeding first rare gas-enriched bottoms liquid to said MP column afterpressure adjustment for separation into second rare gas-depletedoverhead vapor and second rare gas-enriched bottoms liquid; at leastpartially condensing first rare gas-depleted overhead vapor by indirectheat exchange against second rare gas-enriched bottoms liquid to produceat least partially condensed first rare gas-depleted overhead; feedingat least a portion of the at least partially condensed first raregas-depleted overhead to the HP column as reflux; feeding liquid from orderived from the HP column to at least one column selected from the LPcolumn and the MP column as reflux; feeding second rare gas-enrichedbottoms liquid to said LP column for separation into said rare gas-leanGOX and said rare gas-enriched product; at least partially condensingsecond rare gas-depleted overhead vapor by indirect heat exchangeagainst said rare gas-enriched product to produce at least partiallycondensed second rare gas-depleted overhead; feeding at least a portionof the at least partially condensed second rare gas-depleted overhead tothe MP column as reflux; and feeding liquid from or derived from the MPcolumn to the LP column as reflux.
 28. The process according to claim 27further comprising feeding LOX to at least one of the columns as reflux.29. The process according to claim 28 further comprising separating feedair in a cryogenic ASU into nitrogen-rich overhead vapor and LOX andusing a portion of said LOX to provide said LOX reflux.
 30. The processaccording to claim 15 wherein the distillation system comprises at leasta first distillation column and a second distillation column, said firstand second columns operating at the same pressure, said processcomprising: feeding said mixture to said first column for separationinto rare gas-lean GOX and rare gas-enriched product; feeding saidmixture to said second column for separation into rare gas-lean GOX andrare gas-enriched product; feeding LOX to said first column as reflux;and feeding at least one liquid selected from the group consisting ofrare gas-enriched product from the first column and LOX to said secondcolumn as reflux.
 31. The process according to claim 30 furthercomprising dividing a stream of said mixture into two equal portions andfeeding one portion to each column.
 32. The process according to claim 1wherein the concentration of xenon in the mixture feed is no more that50 times greater than the concentration of xenon in air.
 33. The processaccording to claim 1 wherein the concentration of xenon in the mixturefeed is no more that 20 times greater than the concentration of xenon inair.
 34. The process according to claim 1 wherein the concentration ofxenon in the mixture feed is about 5 times greater than theconcentration of xenon in air.
 35. The process according to claim 1wherein the separation is a crude separation and the rare gas-enrichedproduct is further processed to produce at least one product selectedfrom the group consisting of a purified xenon product, a purifiedkrypton product and a purified krypton and xenon product.
 36. Apparatusfor the recovery of at least one rare gas selected from the groupconsisting of krypton and xenon from a pressurized mixture comprisingoxygen and at least one rare gas selected from the group consisting ofkrypton and xenon, said apparatus comprising: a cryogenic ASU forseparating feed air into nitrogen-rich overhead vapor and LOX;pressurizing means for pressurizing at least a portion of said LOX toprovide pressurized LOX; vaporizing means for vaporizing at least about50 mol % of said pressurized LOX to provide said mixture; and a rare gasrecovery system for separating said mixture into rare gas-lean GOX andrare gas-enriched product.
 37. Apparatus according to claim 36 whereinthe rare gas recovery system is a gas-liquid contact column with nodistillation stages for contacting mixture with LOX.
 38. Apparatusaccording to claim 36 wherein the rare gas recovery system is adistillation system.
 39. Apparatus according to claim 38 furthercomprising conduit means for feeding a portion of said pressurized LOXfrom the pressurizing means to the vaporizing means and further conduitmeans for feeding the remaining portion of said pressurized LOX from thepressurizing means to the distillation system as reflux.
 40. Apparatusaccording to claim 38 further comprising heat exchange means for atleast partially condensing a portion of said rare gas-lean GOX overheadagainst a refrigerant to provide at least partially condensed raregas-lean GOX overhead and conduit means for feeding at least a portionof said at least partially condensed overhead to the distillation systemas reflux.
 41. Apparatus according to claim 38 further comprising: afirst distillation column for separating mixture into rare gas-lean GOXand rare gas-enriched product; a second distillation column forseparating mixture into rare gas-lean GOX and rare gas-enriched productat the same pressure as said first distillation column; conduit meansfor feeding LOX to at least one of the distillation columns as reflux;and conduit means for feeding at least one liquid selected from thegroup consisting of rare gas-enriched product from the firstdistillation column and LOX to the second distillation column.